1. Field of the Invention
The present invention relates to a transflective liquid-crystal-display (LCD) device and, more particularly, to a transflective LCD device having a transmissive area and a reflective area in each pixel of the LCD device.
2. Description of the Related Art
LCD devices are roughly categorized into two types including a transmissive LCD device and a reflective LCD device. In general, the transmissive LCD device has a backlight unit and controls the transmission amount of light from the backlight unit to thereby represent an image on the screen. The reflective LCD device has a reflection film that reflects light incident from the outside and uses the light reflected by the reflection film as a light source to represent an image. That is, the reflective LCD device does not require the backlight unit, and is more advantageous than the transmissive LCD device in terms of a reduction in power dissipation, thickness, and weight. However, since the reflective LCD device uses the background light as the light source, visibility of the image is degraded if the background of the LCD device is dark.
As a LCD device having both advantages of the reflective LCD device and transmissive LCD device, a transflective LCD device is known (refer to, e.g., JP-2003-344837A, FIGS. 4 and 20, columns 0009 to 0019, columns 0045 to 0048). The transflective LCD device has a transmissive area and a reflective area in each pixel of the LCD device. The transmissive area transmits the light emitted from a backlight unit and uses the backlight unit as a light source for representing the image. The reflective area has a reflection film and uses the light incident thereto from the outside and reflected by the reflection film as the light source. In the transflective LCD device, if the background of the LCD device is light, the image is represented on the screen by using the reflective area, with the backlight unit being turned off, to thereby achieve a reduction in power dissipation. On the other hand, if the background of the LCD device is dark, the backlight unit is turned on to represent the image by using the transmissive area, thereby enabling the image to be represented on the screen even in a dark background.
A lateral electric-field LCD device such as an in-plane-switching mode (IPS mode) is known as a display mode of the LCD device. The LCD device has a pixel electrode and a common electrode formed in each pixel on the common substrate, and these electrodes apply therebetween a lateral electric field to a liquid crystal (LC) layer. The IPS-mode LCD device rotates the LC molecules in the LCD layer in the direction parallel to the surface of the substrate so as to represent the image, thereby achieving a wirier viewing angle than a twisted-nematie(TN)-mode LCD device.
If the IPS mode is adopted in the transflective LCD device, a problem arises in that the black image and the white image are inverted between both the areas, as described in the technique of the above patent publication. More specifically, if the transmissive area is set at a normally-black mode, the reflective area will have a normally-white mode. The problem of the image inversion will be described hereinafter for a better understanding of the present invention.
FIG. 22A schematically shows the sectional view of the transflective LCD device, wherein a double-sided arrow shows the direction of polarization axis of the polarizing film as viewed parallel to the substrate. FIG. 22B shows the polarized direction of light in both the reflective and transmissive areas 55 and 56 for the structure of FIG. 22A, upon emission of the light at the first polarizing film 51, LC layer 53 and second polarizing film 52. In FIG. 22B, the double-sided arrow depicts the linearly-polarized light, a thick arrow represents the traveling direction of the light, a circled “R” denotes a clockwise-circularly-polarized light, a circled “L” denotes a counterclockwise-circularly-polarized light, and a hollow bar denotes the direction of LC director (molecules). FIG. 22A shows the state of a single pixel including the reflective area 55 and the transmissive area 56. The reflective area 55 uses the reflected light from a reflection film 54 as a light source, and tie transmissive area 56 uses the backlight unit as a light source.
The polarizing film (first polarizing film) 51 on the light emitting side is a common polarizing film effecting on both the reflective and transmissive areas 55 and 56, whereas the polarizing film (second polarizing film) 52 is a dedicated polarizing film effecting on the light incident side of the transmissive area 56. These polarizing films 51 and 52 are arranged such that the polarizing axes thereof cross at right angles.
In the LC layer 53, LC molecules are arranged such that the molecular direction upon absence of applied voltage is shifted by 90 degrees relative to direction of the polarization axis (optical transmission axis) of the second polarizing film 52. Assuming that the direction of the polarization axis of the second polarizing film 52 shown in FIG. 22A is the reference direction, or at 0 degree, the direction of the polarization axis of the first polarizing film 51 is set at 90 degrees and the initial direction of the longitudinal axis of LC molecules in the LC layer 53 is set at 90 degrees, as shown in FIG. 22A.
In the transmissive area of the LC layer 53, the cell gap of the LC layer 53 is adjusted such that the retardation Δnd (Δn is the refractive index anisotropy of LC molecules and “d” is the cell gap) assumes λ/2 (λ is the wavelength of light; for example, in the case of green light, λ is 550 nm) and in the reflective area 55 of the LC layer 53, the cell gap is adjusted such that the retardation assumes λ/4. The image represented on the screen of the LCD device will be described hereinafter in the case of absence and presence of applied voltage for the respective areas 55 and 56.
<Reflective Area Upon Absence of Applied Voltage>
The image in the reflective area 55 upon absence of applied voltage (Voff) on the LC layer 53 will be described first with reference to the leftmost column of FIG. 22B. In the reflective area 55, 90-degree linearly-polarized light passing through the first polarizing film 51 enters the LC layer 53. The direction of the optical axis of the linearly -polarized light that has entered the LC layer 53 and direction of the longitudinal axis of LC molecules are aligned in this case, whereby the 90-degree linearly-polarized light is passed through the LC layer 53 without a change in the polarization and is reflected by the reflection film 54. Thus, the 90-degree linearly-polarized light enters the LC layer 53 once again without a change. The 90-degree linearly-polarized light is thus emitted through the LC layer 53 and enters the first polarizing film 51. Since the direction of the polarization axis of the first polarizing film 51 is set at 90 degrees, the linearly-polarized light is passed through the first polarizing film 51. As a result, a bright image (white image) is represented upon Voff of the LCD device.
<Reflective Area Upon Presence of Applied Voltage>
Next, the state in the reflective area 55 upon presence of applied voltage on the LC layer 53 will be described with reference to the second leftmost column in FIG. 22B. A 90-degree linearly-polarized light passing through the first polarizing film 51 enters the LC layer 53. Here, the applied voltage causes the direction of the longitudinal axis of LC molecules in the LC layer 53 to be changed from 0 degree to 45 degrees with respect to the surface of the substrate. Since the polarized direction of the light that has entered the LC layer 53 is deviated by 45 degrees from the direction of the longitudinal axis of LC molecules and the retardation of the liquid crystal is set at λ/4, the 90-degree linearly-polarized light that has entered the LC layer 53 assumes a clockwise-circularly-polarized light, which enters the reflection film 54. The clockwise-circularly-polarized light is reflected by the reflection film to be changed into a counterclockwise-circularly-polarized state. The counterclockwise-circularly-polarized light that has entered the LC layer 53 passes therethrough once again to be changed into a horizontal (0-degree) linearly-polarized light. The horizontal-linearly-polarized light then enters the first polarizing film 51. Since the direction of the polarization axis of the first polarizing film 51 is at 90 degrees, the light reflected by the reflection film 54 cannot be passed through the first polarizing film 51, with the result that a dark image (black) is represented on the screen.
As described above, the reflective area 55 assumes a normally white mode in which a bright image (white) is represented upon absence of applied voltage (Voff) and a dark image (black) is represented upon presence of applied voltage (Von).
<Transmissive Area Upon Absence of Applied Voltage>
Next, the state in the transmissive area 56 upon absence of applied voltage on the LC layer 53 will be described with reference to the second rightmost column in FIG. 22B. In the transmissive area 56, a horizontal-linearly-polarized light passing through the second polarizing film 52 enters the LC layer 53. The polarized direction of the incident light and longitudinal direction of LC molecules cross each other at right angles, whereby the horizontal-linearly-polarized light is passed through the LC layer 53 without a change in the polarization and enters the first polarizing film 51. Since the direction of the polarization axis of the first polarizing film 51 is at 90 degrees, the transmitted light cannot be passed through the first polarizing film 51, resulting in display of a dark image on the screen.
<Transmissive Area Upon Presence of Applied Voltage>
Next, the state in the transmissive area 56 upon presence of applied voltage on the LC layer 53 will be described with reference to the rightmost column in FIG. 22B. In the transmissive area 56, a horizontal-linearly-polarized light passing through the second polarizing film 52 enters the LC layer 53. Here, the applied voltage causes the direction of the longitudinal axis of LC molecules in the LC layer 53 to be changed from zero degree to 45 degrees with respect to the surface of the substrate. Thus, the polarized direction of the light that has entered the LC layer 53 is shifted to 45 degrees with respect to the direction of the longitudinal axis of LC molecules. Since the retardation of the LC layer is set at λ/2, the horizontal-linearly-polarized light that has entered the LC layer 53 is changed into a vertical-linearly-polarized light and enters the first polarizing film 51. As a result, in the transmissive area 56, the first polarizing film 51 passes the backlight transmitted thereto through the second polarizing film 52, resulting in representing a bright image on the screen.
As described above, the transmissive area 56 assumes a normally black mode in which a dark image is represented upon absence of applied voltage (Voff) and a bright image is represented upon presence of applied voltage (Von).
In the above configuration, the transflective LCD device has a disadvantage in that a dark image and a bright image are inverted between the reflective area 55 and the transmissive area 56 upon both the presence and absence of the applied voltage on the LC layer 53. A technique to solve this disadvantage is described in the above patent publication. FIG. 23 shows a sectional view of the LCD device described in the patent publication. In this technique, the direction of the polarization axis of the first polarizing film 51 is shifted by 45 degrees from the direction of the longitudinal axis of LC molecules in the LC layer 53. The mere arrangement of the polarization axis of the first polarizing film 51 and the longitudinal axis of LC molecules in the LC layer 53 will cause the reflective area 55 using the reflection film 54 as the light source to assume a normally black mode and cause the transmissive area 56 using the backlight unit 57 as the light source to assume a normally white anode. In addition thereto, a λ/2 film 58 is inserted between the second polarizing film 52 and the LC layer 53 to thereby change the transmissive area 56 into a normally black mode, which accords the normally black mode of the reflective area 55.
The direction of the optical axis of the λ/2 film 58 that crosses the direction of the longitudinal axis of the LC layer 53 at right angles is set at 135 degrees. Thus, in front view of the LCD device, optical compensation is achieved wherein the polarization effect that the LC layer 53 having a retardation of λ/2 exerts on the light compensates the polarization effect of the λ/2 film. The optical compensation achieves that the polarized state of light is not changed between the incidence and emission thereof, in consideration of the polarization of light effected by the LC layer 53 and λ/2 film 58 as a whole. Therefore, the light passing through the second polarizing film 52 to assume the horizontal-linear-polarized light is passed through the LC layer 53 and λ/2 film 58 without a change in the polarization and cannot be passed through the first polarizing film 51 having an optical axis set at the vertical direction. That is, the insertion of the λ/2 film 58 between the LC layer 53 and the second polarizing film 52 causes the transmissive area 56 to assume also a normally black mode.
However, in the LCD device 50a shown in FIG. 23, the polarized direction of the light that enters the LC layer 53 and direction of the longitudinal axis of the LC molecules in the LC layer 53 are not parallel or perpendicular to each other in the transmissive area 56. This leads to a disadvantage in that leakage light cannot be suppressed sufficiently, upon display of a dark image, in the transmissive area 56 due to wavelength dispersion characteristics of retardation in the LC layer 53. Further, the λ/2 film 58 also has wavelength dispersion characteristics, thereby causing leakage light due to the wavelength dispersion upon display of a dark image.
In order for solving the above problem, a configuration may be also considered wherein absence of applied voltage in the transmissive area 56 is achieved upon presence of applied voltage in the reflective area by inverting the voltage applied to the transmissive area to thereby apply the inverted voltage to the reflective area. However, such a device scheme or drive technique that can realize this configuration is not known in the art. In addition, problems encountered by this configuration as well as the countermeasures for solving the problems are not known in the art.
Next, in an IPS-mode transflective LCD device, a configuration will be considered in which a first polarizing film, a first λ/2 film, a first λ/4 film, a first LC-layer compensation film (positive or negative λ/4 film), an LC layer, a second LC-layer compensation film (positive or negative λ/4 film), a second λ/4 film, a second λ/2 film, and a second polarizing film are consecutively layered one on another from the light emitting side. In this configuration, each of the first polarizing film, λ/2 film, λ/4 film and second polarizing film, λ/2 film, λ/4 film are so arranged as a broadband λ/4 film.
If the first and second LC-layer compensation films each are a positive λ/4 film, these films are arranged such that the optical axes thereof cross the direction of the longitudinal axis of LC molecules at right angles. On the other hand, if the first and second LC-layer compensation films each are a negative λ/4 film, these films are arranged such that the optical axes thereof axe parallel to the direction of the longitudinal axis of LC molecules. As a result, the LC layer is configured as a λ/2 film.
Accordingly, the effective retardation Δnd of the first and second LC-layer compensation films and LC layer in total assumes 0 in the state of initial orientation of the LC molecules, thereby providing a dark image in a normally black mode in both the transmissive area and reflective area. However, in this configuration, it is impossible to completely perform phase compensation of the polarized light if the birefringence wavelength dispersion differs between the LC-layer compensation film and the LC layer. Further, it is difficult to perform a gap control for the LC layer. As a result, leakage light and/or coloring (chromaticity shift) occurs during display of a dark image. Therefore, the birefringence wavelength dispersion of a material used for the compensation layer and wavelength dispersion of the LC layer need to correspond to each other. Thus, there remains the problem that cannot be solved only by the device configuration.